Ornamental design for a microphone



FIG. 1 is a front view of my new design for a microphone;

FIG. 2 is a rear view of my new design;

FIG. 3 is a left side view of my new design;

FIG. 4 is a right side view of my new design; and _

FIG. 5 is a top view of my new design. 

The ornamental design for a microphone, substantially as shown. 